High speed optical switching device including a capacitance structure

ABSTRACT

A high speed optical switching device includes a waveguide arrangement, incorporating a pn junction, with a large bondpad for the purposes of increasing the capacitance presented by the device. Data and control signals can be input at an input port of the waveguide arrangement, presence of the control signal operating to modify the optical path length of the device for the data signal. By incorporating the device in an interferometer arrangement, or by exploiting a Fabry-Perot cavity of the device, it can be used to transmit the data signal selectively, for instance so as to demultiplex a time multiplexed optical communications signal. The high capacitance provides a very short recovery times so that data rates of 10 GHz and substantially above can be accommodated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical switching devices and findsparticular application in high bit rate communications links.

2. Related Art

Communications links which can carry high data rates are advantageousbecause they can transmit increased levels of information and/or canprovide links via a single physical connection which service highernumbers of customers. For instance, time division multiplexed signals ona communications link can provide a higher number of time slots, andpotentially therefore service a higher number of customers, where thelink itself carries a higher data rate.

In order to access the data on a communications link, it is necessary todown load the information on the link to a receiver. In time divisionmultiplexing, one particular customer will require information from onlyone or more selected time slots to be down loaded. To do this, switchingdevices may be used, the speed of switching of the device beingcommensurate with the capability of the link for carrying high speedtraffic. In optical communications, it is envisaged that data rates maybe achieved as high as 100G bits/s in the foreseeable future. This mightbe carried in ten time slots, providing ten channels at 10G bits/s.

As well as potentially increasing the number of customers using acommunications link, ultra high bit rate links in future networks mayenable customers to be offered new large bandwidth services as well asgiving telecommunications companies greater flexibility in managingtheir networks. The present invention is concerned with an importantpart of achieving ultra high bit rate links, providing a type of switchcapable of demultiplexing a 10G bit/s bit stream from a 100G bit/soptical time division multiplexed (OTDM) signal. An optical clock signalmight be used to switch out every tenth bit. Hence, to recover all thedata on the link, ten switching devices might be used in parallel.

Switching devices are known, including switching devices which can becontrolled by an optical input. For instance, in "130 ps" Recovery ofAll-optical Switching in a GaAs Multi Quantum Well Directional Coupler"published in Applied Physics Letters volume 58 number 19 on 13th May1991 by Li Kam Wa et al (Ref. (i)), a recovery time of 130 ps isreported in a zero gap directional coupler using multiple quantum wells.However, it has now been discovered in making the present invention thatmuch faster recovery times can be achieved. Reduced recovery timesprovide potentially faster switches, recovery time being a limitingfactor.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a semiconductoroptical switching device, for use in optical communications systemsoperating at high data rates, the switching device comprising an opticalwaveguide, including a p-n junction, provided on a substrate, thewaveguide having input and output ports for receiving a data signal at afirst wavelength from a communications link and a control signal at asecond wavelength, the control signal operating to modify the refractiveindex of at least part of the waveguide at the first wavelength, bycreating electrical carriers in the region of the p-n junction, so as tochange the optical path length of the data signal in the device therebeing significant capacitance across the device in use.

The purpose of the significant capacitance is to sweep carriers out ofthe region of the junction as quickly as possible after their creationby input of the control signal to the device. This is achieved becausethe significant capacitance provides a low impedance recombination pathfor photogenerated carriers, thus speeding up the recovery of thedevice, and therefore its potential switching speed.

Although the data and control signal need not both be guided through thedevice, so that for example the data signal could be guided by thewaveguide while the control signal is input at right angles to thewaveguide and is not guided by it, preferably both the data and controlsignal are guided along the waveguides. This facilitates easy alignmentof the device, particularly if the data and control signal share thesame input and output port to the waveguide.

Preferably the significant capacitance of the device enables in use asubstantially constant potential to be maintained across the p-njunction. This is desirable because it is this potential which providesthe driving force to sweep the carriers out of the junction region.

Furthermore the significant capacitance of the device preferablyfacilitates recombination of an AC component of the electrical carrierswithin the device. Hence by arranging for recombination to occur withinthe device the potentially long delays encountered when allowingrecombination to occur outside of the device, due for example to high ACimpedance of the bondwire, can be avoided.

A suitable capacitance can be achieved by providing a higher bondpadcapacitance than would be used in known devices of similar structure.For instance, the latter may have a capacitance of the order of 0.5 pFacross the junction and a bondpad capacitance of 0.1 pF or less. Indeed,in known devices the aim is to reduce the capacitance presented. Devicesaccording to the present invention might, in contrast, present acapacitance in use in the range of, say, 5 pF to 50 pF. For instance,under a reverse bias of 5 volts, the junction alone might provide acapacitance (C_(d)) of 0.6 pF, but the switching device as a whole,including the bondpad capacitance, might present a capacitance (C_(ext))of 23 pF.

The junction can be provided as a PIN junction, absorption of light atthe second (control) wavelength giving rise to electron hole pairs (EHP)as carriers.

Particularly useful signal and control wavelengths might be 1.55 μm and1.3 μm, these being commonly used in optical communications and sourcesof light at these wavelengths being readily available.

A switching device according to an embodiment of the present inventioncan be used as a switch by periodically (selectively) transmitting orblocking the signal wavelength. This might be achieved by putting thedevice into one arm of a Mach-Zehnder interferometer, or by constructingthe device to comprise a Fabry-Perot (FP) cavity, or a directionalcoupler.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described,with reference, by way of example only, to the accompanying drawings ofwhich:

FIG. 1 shows a perspective view of a demultiplexing (DEMUX) chipaccording to an embodiment of the present invention;

FIG. 2 shows a cross section of the waveguiding region of the chip ofFIG. 1;

FIG. 3 shows a circuit model of the DEMUX chip of FIG. 1;

FIG. 4 shows an experimental arrangement for investigating operation ofthe DEMUX chip of FIG. 1;

FIG. 5 shows experimental results obtained with the experimentalarrangement of FIG. 4, in particular the measured ratio of the opticallymodulated signal to an optical modulating signal; and

FIG. 6 shows an alternative embodiment of a DEMUX chip for use in thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring to FIGS. 1 and 2, an embodiment of the present inventioncomprises a PIN waveguide device structure 1, designed to demultiplex a10G bit/s bit stream from a 100G bit/s OTDM signal at 1.55 μmwavelength. Such a switching device might then be used as a DEMUX chipwith an optical clock signal of 1.3 μm wavelength, the clock signalbeing used to switch out, for instance, every tenth bit of the 100Gbit/s signal.

The PIN waveguide device structure 1 is grown by MOVPE growthtechniques, these being known and not consequently further describedherein. The structure is based on a Sn doped InP substrate 2. The layersgrown onto the substrate are as follows, in order:

i) 2×10¹⁸ cm⁻³ S doped n⁻ InP layer 3 index matched to the substrate,

ii) 3×10¹⁷ cm⁻³ S doped n⁻ InP buffer layer 4;

(iii) 0.30 μm quaternary (Q) 1.44 layer 5;

(iv) 0.18 μm undoped InP layer 6;

(v) 8.3×10¹⁷ cm⁻³ Zn doped p⁺ InP layer 7.

Referring particularly to FIG. 1, input and output ports 8, 9 areprovided as facets at either end of the waveguiding structure.Absorption of 1.3 μm light in the Q1.44 guiding layer 5 gives rise toEHPs and an associated change in the refractive index (Δn) of this layer5 for 1.55 μm light. The transmitted intensity at 1.55 μm can then beoptically controlled by putting the device 1 into a Mach-Zehnderinterferometer type of arrangement or by relying on a Fabry-Perotcavity. Ultra high speed operation is made possible by using the newapproach of on chip recombination of swept EHPs, discussed in furtherdetail below. The change in refractive index, Δn, is due to effects ofband filling, band gap shrinkage and free carrier absorption, discussedin the following two papers:

Ref (ii) "Carrier-Induced Change in Refractive Index of InP, GaAs andInGaAsP" IEEE Journal of Quantum Electronics, volume 26 number 1 January1990 by Bennett et al

Ref (iii) "InP/GaInAsP Guided-Wave Phase Modulators based onCarrier-induced Effects, Theory and Experiment" Journal of LightwaveTechnology volume 10 number 1 January 1992 by Vinchant et al.

Practical details of operation of the embodiment of the presentinvention shown in the Figures are as follows:

Pulse Energy Required For Switching:

Besides having the potential for integration with other devicesoperating in the 1.3 μm or 1.55 μm telecommunications windows, usingInGaAsP lattice matched to InP is advantageous in that its bandgap canbe chosen such that Δn will be maximised for a given wavelength. Thebandgap wavelength of 1.44 μm was selected to give a large value of Δndue to the close proximity of this wavelength to 1.55 μm, whilstmaintaining an acceptably small absorption coefficient. For this choiceof bandgap the dominant contribution to Δn is expected to bebandfilling, with the band-gap shrinkage and free carrier absorptionprocesses being smaller and of opposite polarities to one another (seeRef. (ii) above). From consideration of the bandfilling effect alone aproportionality constant relating Δn to the free carrier density ofapproximately 5×10⁻²⁰ cm⁻³ is theoretically predicted (see Ref. (iii,above). A high finesse FP cavity is then used to convert this phasemodulation into intensity modulation of 1.55 μm light. This wasaccomplished by applying facet coatings at the input/output ports 8, 9of reflectivity 84% of 1.55 μm and 8% at 1.3 μm. When these coatingswere applied to a device of length 540 μm a 9.1 dB contrast ratio wasachieved by tuning through the FP spectrum. The estimated contrast ratioobtained when 1 pJ of 1.3 μm light is coupled into the device iscalculated to be 6.1 dB using the proportionality constant above andstandard FP theory (Ref. (iv): "Simple and Accurate Loss Measurement forSemiconductor Optical Waveguides" Electronics Letters Vol. 21 pp 581-583by R G Walker).

Device Switching Speed:

An initial investigation of the free carrier induced Δn using undopedwaveguides indicated that free carriers have a lifetime of approximately8 ns for this material system. In the working device this free carrierlifetime needs to be reduced to <10 ps in order to facilitate 100Gbits⁻¹ operation. For practical purposes the turn-on time can beconsidered to be instantaneous compared to the switch-down time. By theapplication of sufficient reverse bias between the p-contact (bondpad)11 and n-contact (metallized substrate) 12, an electric field can beestablished across the depleted guiding layer 5, 6 resulting in removalof EHP at a rate determined by their transit times out of this layer. Ifwe assume that the saturated carrier velocities of electrons and holeshave the same value as in InGaAs 7×10⁶ cms⁻¹ and 5×10⁶ cms⁻¹respectively, then all carriers will be removed from the depletionregion in <10 ps with the mean carrier transit time being 4.4 ps (Ref.(v): "GaInAsP Alloy Semiconductors" by T P Pearsall, published by Wiley1982).

The idea of reverse biasing an optical switch to sweep carriers out ofits guiding region has been reported before (see Ref. (i) above). Byreverse biasing an MQW structure carriers were swept out of the guidingregion to bring the recovery time down to 130 ps. However the devicestructure reported here is significantly different in the following twoways. Firstly we are employing a bulk semiconductor guiding layer. As aconsequence of this generated carriers don't have to tunnel throughbarriers and hence are not slowed down on their way out of the guidinglayer. The second and more fundamental divergence relates to the currentpulse that is initiated by the removal of EHP from the depletion region.When the carriers are swept out at their saturated carrier velocitiesthis current pulse will be of several hundred mA in magnitude. Atimpedance above a few ohms, it can be seen that potentials of severalvolts could be generated across the external circuit in opposition tothe bias voltage. The DEMUX chip and bias current can be represented bythe circuit element model 10 shown in FIG. 3. When under a reverse biasof 5 V the pin junction has a leakage current of 370 pA and acapacitance Cd=0.6 pF. The resistance 13 associated with the path fromthe metallized ridge to the bondpad 11 is approximately R=1.6Q. Thelarge area bondpad 11, fabricated using a 0.1 μm SiNx dielectric layer,has a capacitance at -5 V of C_(ext) =23 pF. When the 1.3 μm clock pulsehas a repetition rate of 10 GHz the current generated will consist of aDC component in addition to AC components going up to in excess of 100GHz in 10 GHz steps. DEMUX chips were bonded to laser headers tofacilitate the application of a bias voltage. There will inevitably be abondwire inductance L, 14, of several nH acting in series with variousexternal resistances R. As long as R is less than a few ohms the DCcomponent of the current will recombine off chip without dropping thejunction potential by a significant amount. The case is different forthe AC components due to the large impedance of the bondwire at thesefrequencies. This necessitates on chip recombination which is madepossible by the high value C_(ext). For the value given above assumingthat any inductance in series R is of negligible size, 100G bits⁻¹operation is predicted for low reverse voltages.

The idea of increasing the bondpad capacitance to improve high speedperformance is directly opposite to the approach required forphotodetectors and electrooptic modulators, and is what sets this deviceapart from previous work.

Experiment:

Using the lay-out shown in FIG. 4, a CW beam from a 1.56 μm DFP 15 waswavelength tuned so that the transmitted TE mode from the reverse biasedDEMUX chip 1 was roughly at a point in its FP spectrum where the changein transmitted intensity for small Δn was optimised. This beam was thenmodulated in the DEMUX chip 1 by amplified 1.3 μm pulses ofapproximately 25 ps FWHM duration obtained by gain switching a DFB 16 ata 1 GHz repetition rate. The signal was detected by a high speed pinphotodector 17, amplified, and then displayed on an rf spectrum analyser18. At each frequency the signal level was calibrated against the 1.3 μmsignal at that frequency. The spectrum of the 1.3 μm pulse was observedby connecting the output of the WDM coupler 19 to the high speed pin 17.Measurements were conducted in the frequency domain because in the timedomain using this source the response would be dominated by the spectralcomponents outside the range of interest.

Results and discussion:

With the DEMUX chip 1 under a 1 V reverse bias optical modulation wasobserved at frequencies up to 20 GHz. Using this measurement approachthere was a restriction imposed by the spectrum of the 1.3 μm pulsewhich falls off at around 20 GHz. The measured ratio of the 1.5 μmsignal to the 1.3 μm signal is displayed in FIG. 5. It can be seen thatin the frequency range of interest (>10 GHz) the ratio of the modulatedto modulating signal remains approximately constant. Since the impedanceof the current recombination path varies with frequency, this suggeststhat over this frequency range the impedance is sufficiently low so asnot to slow the free carriers down from their saturated carriervelocities. The observed optical modulation of the 1.56 μm signal islarger by a factor of 5 than that theoretically predicted from the levelof the 1.3 μm signal if a mean carrier lifetime of 4.4 ps and a Δnproportionality constant of 5×10⁻²⁰ cm³ are assumed. Below 10 GHz theratio of modulated to modulating signal increases with decreasingfrequency. This has yet to be accurately modelled, but it is believed tobe related to the higher recombination path electrical impedances atthese frequencies. If the impedance of the path between Cd and C_(ext)could be made to be negligible, then this design could function as anoil optical wavelength converter besides its envisaged role as ademultiplexer.

DEMUX chips using the novel swept carrier technique could be used todemultiplex an ultra fast bitstream by employing either the high finesseFP cavity technique or the more familiar Mach-Zehnder approach. When theFP cavity method is employed the amount of energy required in theswitching pulse is less. However the linewidth of the 100G bits⁻¹bitstream will be at least 0.8 nm which is not negligibly small incomparison with the spacing of adjacent maxima of the DEMUX FP spectrum.This fact when taken into consideration alongside the requirement tokeep the mean photon lifetime in the cavity down to values less than thebit period implies that there is little benefit to be accrued fromreducing the device length or absorption loss to values below thosereported here.

Referring to FIG. 6, an alternative form of DEMUX chip for use inembodiments of the present invention comprises a ridge 60 flanked by adouble channel arrangement 61, 62. The upper surface of the chip 1a ismetallized 63 on top of a thin (1000 Å) nitride layer. Electricalcontact is made to the ridge region only. The layer structure isotherwise the same as that shown in FIG. 1.

The double channel structure may be used with a variety of channel etchdepths, for instance, or other variations. An example is that etchingcould stop part of the way through, or above, the guiding layer 5.

The form of DEMUX chip shown in FIG. 6 has a particularly simplestructure and is thus easily fabricated with high yield.

We claim:
 1. A semiconductor optical switching device, for use inoptical communications systems operating at high optical data rates, theswitching device comprising:an optical waveguide, including a p-njunction, provided on a substrate, the optical waveguide having inputand output ports for receiving an optical data signal at a first opticalwavelength from a communications link and an optical control signal at asecond optical wavelength, the optical control signal operating tomodify the refractive index of at least part of the waveguide at thefirst optical wavelength, by creating electrical carriers in the regionof the p-n junction, so as to change the optical path length of the datasignal in the device, there being capacitance structure which providessignificant capacitance substantially in excess of 0.6 pf across thedevice in use.
 2. A device according to claim 1, wherein both theoptical data and optical control signal are guided along the opticalwaveguide.
 3. A device according to claim 1, wherein the significantcapacitance of the device enables, in use, a substantially constantreverse-bias potential to be maintained across the p-n junction.
 4. Adevice according to claim 1, wherein the significant capacitance of thedevice, in use, facilitates recombination of an AC componentreverse-bias p-n junction current due to the electrical carriers withinthe device.
 5. A device according to claim 1 wherein said opticalswitching device has a recovery time of not more than 100 psecs.
 6. Adevice according to claim 1 wherein said optical switching device has arecovery time of not more than 10 psecs.
 7. A device according to claim1 wherein the significant capacitance of the optical switching device isprovided by a large area bondpad.
 8. A device according to claim 7having a ridge structure flanked on each side by a channel, wherein:saidwaveguide is located within the ridge structure, and said large areabondpad is provided substantially over the whole of the ridge structureand channels.
 9. A device according to claim 1 wherein said significantcapacitance has a value of at least 5 pF.
 10. A device according toclaim 1 wherein said high optical data rates comprise modulation atfrequencies of 10 GHz and above.
 11. A device according to claim 1wherein said optical waveguide is within a Fabry-Perot cavity.
 12. Adevice according to claim 11, wherein the facet reflectivity of theFabry-Perot cavity is greater than the first optical wavelength than atthe second optical wavelength.
 13. A demultiplexing arrangement, for usein demultiplexing time division multiplexed optical communicationssignals, comprising an optical switching device according to claim 1.